Conventional Solar Panels
Photovoltaic (PV) cells produce direct current (DC). DC output of PV cells is generally inverted to alternating current (AC). In conventional PV power generation, the DC outputs from a few PV modules (each module or panel producing 24-50V DC potential) are generally connected in series (string) to feed a centralized inverter.
More recently micro-inverters have been used in place of central inverters. A micro-inverter converts the output of a single PV panel to AC. The AC output of multiple micro-inverters may be combined.
Generally, a PV panel is constructed of substrings. Each substring is composed of ten to twenty solar cells in series, and each cell operates at approximately 0.6V. A PV panel generally includes between two to five substrings yielding a panel output of 24V to 60V DC.
Partially Shaded Substrings and Reverse Bias
When a PV panel is partially irradiated such that a few cells are shaded and many cells are fully irradiated, the irradiated cells force the shaded cells to operate in reverse bias mode. In reverse bias, instead of generating electrical energy, a shaded cell dissipates excess power as heat. Heating may lead to a local short-circuit and permanent damage in the cell. In popular crystalline PV modules a bypass diode is used across each substring to prevent such reverse biasing.
FIG. 1 shows a panel with three substrings 14a, 14b, and 14c each having fourteen cells (shown as small square solar batteries). One cell of substring 14c is blocked by shade 16. Each substring 14a-c has a bypass diode 12a, 12b, and 12c respectively. Flow of electricity is shown by dotted arrows. Electricity flows through substrings 14a and 14b but because substring 14c is partially shaded, the voltage potential is small and electricity bypasses substring 14c and passes through bypass diode 12c. 
FIG. 2 shows a current voltage curve, IV-characteristic 21a, of a fully irradiated solar cell in its normal forward operating mode and power 23a generated by the cell at the maximum power point (MPP) 25. Also shown is the IV-characteristic 21b of a shaded cell in forward operating mode.
Because all of the substrings of the PV panel are connected in series, all cells are forced to operate at the same current (substring current). It can be seen that the current 22a produced by the sunlit cell at its MPP is greater than the maximum current 22b produced by the shaded cell. Therefore when a shaded cell is connected in series with a sunlit cell working at high current, the shaded cell becomes reverse biased and begins to dissipate power by heating up.
Cell Breakdown (Burn Out)
FIG. 3 shows a typical IV-characteristic 21d of a shaded solar cell as in FIG. 2 and also illustrates the reveres bias mode (for reverse or negative current). Whereas the forward characteristic extends to the open circuit voltage of approximately 0.6 Volts, reverse biased IV-characteristic 21d is much more extensive and limited by the breakdown voltage threshold 37. This means that one shaded cell may dissipate very large amounts of power 23c, thereby absorbing the power produced by a few irradiated cells.
At low reversed bias voltages the power dissipation is distributed over the whole shaded cell area and heating takes place uniformly. The cell is designed so when the current density is below a critical limit, the cell is stable against thermal effects. With rising reverse bias current a junction breaks down and conducts very large currents. Cells do not have a homogeneous structure, and contain regions with a higher concentration of impurities. At high reverse bias currents these regions break down earlier. If the current density in a high impurity region exceeds a critical limit, the cell is irreversibly damaged by thermal breakdown that forms a shunt path in the cell structure. When a long series of fully irradiated cells is connected in series with a shaded cell, the irradiated cells can produce enough power to burn out the shaded cell.
The process of cell short circuiting is described in HERMANN, Wiesner, et al. Hot Spot Investigations on PV Modules—New Concepts for a Test Standard and Consequences for Module Design with respect to Bypass Diodes. Photovoltaic Specialists Conference 1997, Conference Record of the Twenty-Sixth IEEE 1997, vol. 29, p. 1129-1132, and also in HERRMANN, W, et al. Operational Behaviour of Commercial Solar Cells Under Reverse Biased Conditions. TÜV RHEINLAND SICHERHEIT UND UMWELTSCHUTZ GMBH AM GRAUEN STEIN. 2000.
FIG. 4 illustrates use of a by-pass diode to prevent reverse bias breakdown in a prior art solar panel having three substrings of eighteen cells each connected in series. A bypass diode is used across each substring. When reverse biasing reduces the voltage of a partially shaded substring beyond a danger threshold the diode short circuits the substring. This short circuiting prevents reverse bias that may harm shaded cells in the substring, but leaves the partially shaded substring working at the danger threshold at which no power is generated.
Power Generated by a Partially Shaded Panel
In FIG. 4 the horizontal axis is voltage and the vertical axes of the upper graph is current for IV-characteristics 421a, 421b and 421c; where IV-characteristic 421a is for a panel having one partially shaded substring with a bypass diode, and IV-characteristic 421b is for a panel having one partially shaded substring without a bypass diode and IV characteristic 421c is for a fully irradiated panel. Under full sun and below the MPP voltage, the panel acts as a constant current source with IV-characteristic 421c and maximal power 425c output of about seventy watts at 2.7 amps current.
The lower graph shows power output curves 423a, 423b and 423c voltage vs. power output (watts). Power output curve 423a shows the power output of the panel with one shaded cell with bypass diodes, power output curve 423b shows the power output of a panel with one shaded cell without bypass diodes, and power output curve 423c shows the power output of a fully irradiated panel. It can be seen that the bypass diodes protect the shaded cell from reverse breakdown, but do not significantly help the power output. The activation of the diode in its conductive mode adds a new global peak power 425a of 45 W to the overall partially shaded panel IV-characteristic 423a. The maximum power 425b 38 W of a partially shaded panel without bypass diodes (panel IV-characteristic 423b) is only a local maximum for a panel with bypass diodes (IV-characteristic 423a).
The reduction in power harvesting from a PV panel in serial connection is not insignificant in shading and dynamic irradiation condition and can contribute to a loss of 30% from the potentially available power, for a 5% shaded PV panel. In a situation where the shade is distributed between two substrings the loss of power can amount to 60%. More particularly, at high current the partially shaded panel produces little power because the shaded cell dissipates a lot of power at high current. At low current the partially shaded panel produces little power because the irradiated strings are working far from their MPP.
Some Attempted Solutions
US published patent application US 20090020151 A (FORNAGE) 22 Jan. 2009 (Formage '151) discloses a method to optimize power output from a solar panel by connecting multiple nano-inverters to the panel (for example one nano-inverter for each row of cells). In this way each nano-inverter may be connected to all sunlit cells or to all shaded cells avoiding the problem of partially illuminated substrings. In this way Formage '151 extracts power from substrings that are in the shade along with fully lit substrings. Nevertheless, the method of Formage '151 does not offer a solution to a partially shaded substring. Because the angle of the sun changes both east to west (over the course of a day) and north to south (over the course of a year) it is may not be practical to find a geometry which will never have partially shaded substrings.
US published patent application US 20100106438 (FORNAGE) 29 Apr. 2010 (Formage '438) discloses a controller programmed to compute the MPP and the voltage lower bound for PV cell reverse bias breakdown. The operating voltage is then chosen to be greater than the lower bound and as close as possible to the MPP. The methodology of Formage '438 has a few drawbacks. Firstly, the controller of Formage '438 needs to be much more complex than a standard controller in order to compute both the MPP and the lower bound voltage. Furthermore, the lower bound voltage is a complicated function of temperature, the kind of cells, the quality of the materials used in the cells and the quantity of cells in the panel. This leads to a more complex and expensive solar panel and a less flexible system.
Thus, there is a recognized need and it would be desirable to develop a solar panel which is not vulnerable to reverse bias burn out, extracts global maximum power from partially shaded substrings and is simple to build, operate and repair.
Application to Solar Fields
One problem when designing large solar fields is the transfer of energy from a large array of solar panels to a single collection circuit. Conventional DC series connections require long cables connecting large numbers of panels over a large area. With a central Inverter, the wiring must be carefully balanced in order that the MPP determined by the centralized controller will be correctly and equally distributed to all of the panels. This requires heavy high current DC connections across the field. The complexities of balancing input to the collecting circuit and the cost of cables and their specialized installation and upkeep can be a significant problem. This problem is exacerbated as the field ages because aging affects different components differently and power output from different sets of panels that was originally balanced becomes unbalanced over time causing problems in combining the power and eventually power losses. The delicate balance of various components can also be thrown off by partial shading due to dust and clouds.
Two other technical limitation result from the need to prevent partial shading in large solar installations. Firstly, the distance between rows of panels is kept large. Commonly the distance between rows is equal to the row width (distance 1480a equals distance 1480b in FIG. 14a). Otherwise at times of low angle 1486 solar radiation 1482a (the morning or afternoon) one row 1484a will partially shaded the next row 1484b. Secondly, wiring is installed underground because overhead wiring could cause shading on a few percent of a panel's surface and throw off the MPP or cause activation of burn out protection diodes of the entire installation causing a few tens of percent loss of power.